A so-called polymer electrolyte fuel cell (Polymer Electrolyte Fuel Cell: hereinafter called “PEFC” as needed), has its operating temperature of from a room temperature to about 80° C. Also, since PEFC makes it possible to employ inexpensive general-purpose plastics, etc. for members constituting its fuel cell body, it is possible to realize reduction in weight. Furthermore, PEFC makes it possible to achieve thinning of a polymer electrolyte membrane, enabling an electric resistance to be reduced, thereby enabling a power loss to be reduced relatively easily. Due to PEFC having not a few advantages as described above, it is applicable to a fuel cell vehicle, a home cogeneration system, and the like.
As an electrode catalyst for PEFC, there has been proposed an electrode catalyst in which platinum (Pt) or a platinum (Pt) alloy, i.e., a component for the electrode catalyst, is supported on a carbon serving as a support (for example, Japanese Patent Application Publication No. 2011-3492, MATSUOKA et al., “Degradation of Polymer Electrolyte fuel cells under the existence of anion species”, J. Power Sources, 2008 May 1, Vol. 179 No. 2, P.560-565).
Conventionally, there have been disclosed that, as for an electrode catalyst for PEFC, if the content of chlorine contained in the electrode catalyst is 100 ppm or more, it is not desirable as an electrode catalyst (for example, Japanese Patent No. 4,286,499); and that this is because if the content of chlorine contained in the electrode catalyst is 100 ppm or more, it is impossible to obtain a sufficient catalytic activity for the electrode catalyst for fuel cells; and corrosion of its catalyst layer will occur, thus shortening the life of the fuel cell.
Then, there is disclosed, as the catalyst component of the electrode catalyst, a powder of platinum (Pt) or platinum (Pt) alloy that contains less than 100 ppm of chlorine (for example, Japanese Patent No. 4,286,499).
As for the preparation of the powder of platinum (Pt) or platinum (Pt) alloy, there is disclosed the following method: forming a melt which contains, as starting materials, a chlorine-free platinum compound and a chlorine-free compound of alloying elements; heating the melt up to a reaction temperature at which the platinum compound and the compound of the alloying elements are thermally decomposed to give an oxide; cooling the melt; and the melt is dissolved in water and the resulting oxide or mixed oxides are converted into a powder of platinum or platinum alloy by successive reduction.
Further, there is disclosed a PEFC in which part of protons of an acid group of an electrolyte contained in a catalyst layer of an electrode of a membrane-electrode assembly is exchanged for a phosphonium ion, defining a compound structure such that a counter anion of the phosphonium contains no halogen elements (for example, Japanese Patent No. 5,358,997). The reason, as is disclosed therein. is that residues of the halogen elements in the electrode cause a degradation in cell performance. Specifically, it is described that the residues of a fluoride ion, a chloride ion, or a bromide ion amongst halide ions in the electrode sometimes cause degradation in cell performance, and particularly, the residues of the chloride ion in the electrode poison the electrode catalyst, and cause Pt serving as a catalyst to be eluted from a catalyst layer as a complex ion such as PtCl42−, PtCl62− to cause degradation of the cell performance.
Moreover, there is disclosed a method for producing core-shell particles obtained by filtering a dispersion liquid having core-shell particles dispersed in a solvent using ultrafilters or the like, cleaning and substituting the same with a solvent as necessary (for example, Japanese Patent No. 5,443,029). Specifically, there is disclosed in a preparation process of a core metal particles dispersion liquid, the core metal particles dispersion liquid was cleaned until no Cl ions were detected.
Further, there is disclosed a method for producing carbon supported core-shell catalyst fine particles which controls a deposition of a shell metal material composing a shell part on a surface of a carbon support (for example, Japanese Patent No. 5,672,752). Furthermore, there is disclosed a method for producing a platinum core-shell catalyst capable of directly depositing platinum on a gold core particle (for example, Japanese Patent No. 5,660,603). In production processes provided in these two methods for producing a electrode catalyst (core-shell catalyst), it is disclosed that the electrode catalyst (core-shell catalyst) is cleaned with extra pure water.
However, the methods for producing an electrode catalyst (core-shell catalyst) disclosed in the aforementioned patent documents focus on chlorine only amongst halogens, and merely work on reduction/removal of chlorine although it shows the findings that residues of halogens reduce cell performance.
The applicant of the present patent application presents the following publications as those that describe the aforementioned inventions known to the public through publications.